Conventionally, a nitride semiconductor layer fabricated by a metal organic chemical vapor deposition method (MOCVD method) and doped with p-type impurities was electrically neutral and could not obtain a p-type electric property, and had significantly high resistance. Accordingly, Japanese Patent Laying-Open No. 05-183189 (Patent Document 1) describes that, after the completion of crystal growth, the nitride semiconductor layer doped with p-type impurities and having significantly high resistance is subjected to an annealing treatment in an atmosphere that contains substantially no hydrogen and at a temperature of at least 400° C., to thereby eliminate hydrogen from the nitride semiconductor layer, so that a nitride semiconductor having p-type conductivity and low resistance can be obtained from a nitride semiconductor having high resistance.
Meanwhile, Japanese Patent Laying-Open No. 09-129929 (Patent Document 2) and Japanese Patent Laying-Open No. 2004-103930 (Patent Document 3) describe a manufacturing method for obtaining a high p-type carrier concentration or low resistivity after the completion of crystal growth even without an annealing treatment for obtaining a p-type property.
According to the manufacturing method disclosed in Patent Document 2, when a p-type cladding layer and a p-type contact layer are stacked, for example, they are fabricated under the condition that an inert gas is excessive with respect to hydrogen and a flow quantity ratio of hydrogen to the inert gas is at least 0.75%, and after the completion of crystal growth, a substrate is naturally cooled with a flow quantity of the inert gas increased. Here, in the stage where the flow quantity of the inert gas is increased, a flow quantity ratio between the inert gas and ammonia is set to 2:1. According to this document, such a manufacturing method enables electrical activation of at least 7% of p-type impurities elements, and obtainment of a significantly high carrier concentration of at least 2.4×1018 cm−3.
Furthermore, according to the manufacturing method disclosed in Patent Document 3, a GaN-based semiconductor crystal containing p-type impurities is grown in a crystal growing atmosphere containing an inert gas at a ratio of at least 50 vol %, and after the completion of crystal growth, the atmosphere is replaced with a cooling atmosphere containing ammonia at a ratio of 0.1%-30 vol % at the crystal growth temperature, and the semiconductor crystal is cooled in the cooling atmosphere. According to this document, such a manufacturing method enables a treatment for activating p-type impurities to be stably performed without a heat treatment after growth.
Patent Document 1: Japanese Patent Laying-Open No. 05-183189
Patent Document 2: Japanese Patent Laying-Open No. 09-129929
Patent Document 3: Japanese Patent Laying-Open No. 2004-103930